Stiffened plate and method of manufacturing the same

ABSTRACT

A stiffened plate is formed by welding a plurality of stiffener materials which, together with a steel plate, form closed cross-sectional structures onto a surface of the steel plate. The stiffener materials are joined to the steel plate as a result of edge portions of the stiffener materials which are in contact with the steel plate by being laser welded at a predetermined welding speed as a result of a laser having a predetermined output being irradiated from a predetermined direction thereon from the external side of the closed cross-sectional structures. Accordingly, it is possible to achieve an improvement in the weld quality when welding stiffener materials onto a steel plate, and to achieve an improvement in fatigue strength.

TECHNICAL FIELD

The present invention relates to a stiffened plate and to a method ofmanufacturing a stiffened plate.

Priority is claimed on Japanese Patent Application Nos. 2007-034826,filed Feb. 15, 2007, 2007-098323, filed Apr. 4, 2007, and 2008-018968,filed Jan. 30, 2008, the contents of which are incorporated herein byreference.

BACKGROUND ART

Various materials such as concrete materials and the like are used asthe floor material of bridges and the like. Steel floor plates are knownas one of these materials. Steel floor plates are principally formed bysteel plates (i.e., deck plates), however, deck plates themselves cannot satisfactorily guarantee the necessary strength. Therefore,normally, steel floor plates are formed as stiffened plates which havestiffener materials (i.e., ribs) provided on their bottom surface.

These stiffener materials may be belt-shaped plate steel, or may besteel materials having a V-shaped or U-shaped cross section which areknown as closed cross section ribs or the like. These stiffenermaterials are joined to the bottom surface of a deck plate in parallelwith each other and at fixed intervals so as to form a stiffened plate.

Here, edge portions on both ends of the closed cross section ribs arejoined by arc welding to the deck plate so as to form a closedcross-sectional structure with the deck plate.

A variant example of the above described type of stiffened plate is thesandwich panel shown in Non-patent document 1. This sandwich panel ismanufactured by arranging a number of U-shaped rib members in parallelwith each other on a deck plate so as to form a closed cross-sectionalstructure, and by then arc welding edge portions of the U-shaped ribmembers onto the deck plate. In addition, a bottom plate is placed onapex portions of the U-shaped rib members, and the apex portions of theU-shaped rib members and the bottom plate are joined together by laserwelding.

Note that Patent document 1 below by the applicants of the presentinvention provides documentation for prior art relating to weldingtechnologies for stiffened plates.

Non-patent document 1: “Strength Characteristics of Laser Lap WeldedJoins in Medium Thickness Plates and Panel Manufacturing”, by ShoichiKitagawa, Hitachi Zosen Corporation in “Laser Symposium: Applications ofLaser Welding to Medium Thickness Plate manufactured Bodies”Corporation: Japan Welding Engineering Society, LMP Committee, convokedAug. 22 (Th) to 23 (We), 2002

Patent document 1: Japanese Unexamined Patent Application, PublicationNo. 2006-224137

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In arc welding, since a welded metal experiences thermal contractionafter welding, tensile residual stress (i.e., welding residual stress)is generated within the deck plate and closed cross section ribs. Thiswelding residual stress causes the assembly accuracy of the componentsto deteriorate, giving rise to problems such as deteriorated tensilestrength, compressive strength, or fatigue strength in the components.

Moreover, in the welding of the closed cross-sectional structure portionof a closed cross section rib, it is only possible to perform thewelding operation from the external side of the closed cross-sectionalstructure portion. However, when this type of closed cross-sectionalstructure portion is arc welded, if an attempt is made to preventmelt-through (i.e., over-welding), the problem exists that partiallyunwelded portions (i.e., welding residue portions) occur in the distalend of a single bevel or single-J bevel, namely, in a root portion onthe internal side of the closed cross-sectional structure. Namely, inarc welding, because it is not possible to accurately control the meltrange, if an attempt is made to prevent melt-through, then weldingresidue portions end up being generated.

In this manner, if welding residual stress is generated within a deckplate and closed cross section ribs, and if welding residue portions arepresent in root portions on the internal side of the closedcross-sectional structures, then stress is concentrated in an areadirectly above the welding residue portions of the deck plate. Becauseof this, cracks are easily generated in areas directly above the weldingresidue portions of the deck plate. If cracks are generated in a deckplate, the problems arise that there is an abrupt drop in fatiguestrength and that this might easily lead to fatigue failure. Moreover,because melt-through causes the weld quality to deteriorate, this is notpreferable.

Furthermore, although the laser lap welding described in theaforementioned Non-patent document 1 in which welding is performed so asto penetrate two components in a sandwich panel is the method used forthe laser welding in order to manufacture a stiffened plate, laserwelding is not used conventionally for joining edge portions of theclosed cross section ribs to the deck plate.

DISCLOSURE OF THE INVENTION

The present invention was conceived in view of the above describedcircumstances and it is an object thereof to achieve an improvement inweld quality when welding stiffener materials onto a steel plate, and toachieve an improvement in fatigue strength.

Means for Solving the Problem

In order to achieve the above described object, a first aspect of thepresent invention is a stiffened plate which is formed by welding aplurality of stiffener materials which, together with a steel plate,form closed cross-sectional structures onto a surface of the steelplate, wherein the stiffener materials are joined to the steel plate asa result of edge portions of the stiffener materials which are incontact with the steel plate by being laser welded at a predeterminedwelding speed as a result of a laser having a predetermined output beingirradiated from a predetermined direction thereon from the external sideof the closed cross-sectional structures.

A second aspect of the present invention is the stiffened plateaccording to the first aspect, wherein the edge portions of thestiffener materials are formed so as to face the surface of the steelplate substantially parallel therewith, and are laser welded to thesteel plate.

A third aspect of the present invention is the stiffened plate accordingto the first or second aspects, wherein the stiffener materials areclosed cross section ribs which have a substantially U-shaped crosssection orthogonally to the longitudinal direction.

A fourth aspect of the present invention is the stiffened plateaccording to any one of the first through third aspects, wherein insteadof laser welding, the stiffener materials are welded to the steel plateby means of hybrid laser-arc welding in which laser welding and arcwelding are performed in combination.

A fifth aspect of the present invention is a method of manufacturing astiffened plate which is formed by welding a plurality of stiffenermaterials which, together with a steel plate, form closedcross-sectional structures onto a surface of the steel plate, whereinthe steel plate and the stiffener materials are joined together as aresult of edge portions of the stiffener materials which are in contactwith the steel plate being laser welded at a predetermined welding speedas a result of a laser having a predetermined output being irradiatedfrom a predetermined direction thereon from the external side of theclosed cross-sectional structures.

A sixth aspect of the present invention is the method of manufacturing astiffened plate according to the fifth aspect in which, prior to thelaser welding, the edge portions of the stiffener materials are formedso as to face the surface of the steel plate substantially paralleltherewith.

A seventh aspect of the present invention is the method of manufacturinga stiffened plate according to the fifth or sixth aspects, wherein inthe laser welding, a refractive laser welding apparatus which refractslaser light which has been converged by a convex lens onto a planarmirror is used to irradiate laser light over the surface of the steelplate so as to join the stiffener materials to the steel plate.

An eighth aspect of the present invention is the method of manufacturinga stiffened plate according to any one of the fifth through seventhaspects, wherein in the laser welding, a refractive laser weldingapparatus which converges and refracts laser light using a parabolicmirror or spherical mirror is used to irradiate laser light over thesurface of the steel plate so as to join the stiffener materials to thesteel plate.

A ninth aspect of the present invention is the method of manufacturing astiffened plate according to any one of the fifth through seventhaspects, wherein, the welding is achieved by means of hybrid laser-arcwelding in which arc welding is performed in a process eithersimultaneously with or consecutively to the laser welding.

A tenth aspect of the present invention is the method of manufacturing astiffened plate according to any one of the fifth through ninth aspects,wherein in the laser welding or in the hybrid laser-arc welding, fillerwire is used.

An eleventh aspect of the present invention is the method ofmanufacturing a stiffened plate according to any one of the fifththrough tenth aspects, wherein in the laser welding or in the hybridlaser-arc welding, a bevel shape in which there is no gap with the skinplate is used.

A twelfth aspect of the present invention is the method of manufacturinga stiffened plate according to any one of the fifth through eleventhaspects, wherein in the laser welding or in the hybrid laser-arcwelding, the skin plate is used in a structure having a curved surface.

EFFECT OF THE INVENTION

According to the present invention, because edge portions of stiffenermaterials are joined to a steel plate by laser welding or by hybridlaser-arc welding, for example, by appropriately setting the laseroutput, the irradiation direction, and the welding speed, it is possibleto eliminate the occurrence of melt-through and welding residueportions. Accordingly, it is possible to achieve an improvement in theweld quality when welding stiffener materials onto a steel plate, and toachieve an improvement in fatigue strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the structure of a stiffened platein an embodiment of the present invention.

FIG. 2 is a perspective view showing a portion of the stiffened plateshown in FIG. 1 which has been enlarged and vertically inverted.

FIG. 3 is a cross-sectional view taken along a line A-A in FIG. 2.

FIG. 4 is a typical view showing the structure of a refractive laserwelding apparatus which is used for the laser welding in an embodimentof the present invention, and also shows a state in which thisrefractive laser welding apparatus is in use.

FIG. 5 is a typical view showing a configuration of an edge portion of aclosed cross section rib in an embodiment of the present invention.

FIG. 6 is an explanatory view showing a manufacturing process for asteel floor plate in an embodiment of the present invention.

FIG. 7 is a typical view showing the structure of a refractive laserwelding apparatus which is used in another embodiment of the presentinvention, and also shows a state in which this refractive laser weldingapparatus is in use.

FIG. 8A is a perspective view showing an outer appearance of a closedcross section rib in another embodiment of the present invention.

FIG. 8B is a perspective view showing an outer appearance of a closedcross section rib in another embodiment of the present invention.

FIG. 8C is a perspective view showing an outer appearance of a closedcross section rib in another embodiment of the present invention.

FIG. 8D is a perspective view showing an outer appearance of a closedcross section rib in another embodiment of the present invention.

FIG. 9A is a cross-sectional view illustrating hybrid laser-arc weldingin another embodiment of the present invention.

FIG. 9B is a plan view illustrating hybrid laser-arc welding in anotherembodiment of the present invention.

FIG. 10 is an explanatory view illustrating hybrid laser-arc welding inanother embodiment of the present invention, and is an enlargement of aprincipal portion of FIG. 9A.

FIG. 11A shows a state of a melt portion which corresponds to anirradiation angle θ of a laser L in FIG. 10, and shows a case in whichthe irradiation angle θ of the laser L is not less than 10°.

FIG. 11B shows a state of a melt portion which corresponds to anirradiation angle θ of a laser L in FIG. 10, and shows a case in whichthe irradiation angle θ of the laser L is less than 10°.

FIG. 12A shows a state of a melt portion in FIG. 10 when a Z targetposition is not more than 5 mm (i.e., low).

FIG. 12B shows a state of a melt portion in FIG. 10 when a Z targetposition is greater than 5 mm (i.e., high).

FIG. 13 is an explanatory view illustrating hybrid laser-arc welding inanother embodiment of the present invention, and is an enlargement of aprincipal portion of FIG. 9B.

DESCRIPTION OF THE REFERENCE NUMERALS

-   10 Deck plate-   20 Closed cross section rib-   20 Rib-   21 Edge portion-   22 Edge portion-   22 Laser welding apparatus-   31 Light source-   32 Convex lens-   33 Planar mirror-   34 Laser emission port-   35 Housing-   40 Laser welding apparatus-   41 Light source-   43 Curved mirror-   44 Laser emission port-   45 Housing a: arrow-   L Laser-   M Torch-   t1 Predetermined plate thickness-   t2 Predetermined plate thickness-   W Filler wire-   X Distance from head-   Y Welding direction-   θ Irradiation angle

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference made to the drawings. Firstly, FIG. 1 is a perspectiveview showing the structure of a stiffened plate in the presentembodiment. FIG. 2 is a perspective view showing a portion of thestiffened plate shown in FIG. 1 which has been enlarged and verticallyinverted. FIG. 3 is a cross-sectional view taken along a line A-A inFIG. 2.

Note that stiffened plates are used in a variety of applications such asthe decking and structural components of ships and the like, however,here, an example is described in which stiffened plates are used, forexample, for the steel floor plates of a bridge.

Steel floor plates (i.e., stiffened plates) are formed by placing aplurality of closed cross section ribs 20 (i.e., stiffener materials) atfixed intervals on one surface of a deck plate 10 (i.e., a steel plate).The deck plate 10 is a flat plate made from steel having a predeterminedthickness of t1 (for example, 12 mm). The closed cross section ribs 20are made of shape steel having a U-shaped cross-section and are formedby bending a flat plate made from steel having a predetermined thicknessof t2 (for example, 6 to 8 mm).

FIG. 4 is a typical view showing the structure of a laser weldingapparatus 30 (i.e., a refractive laser welding apparatus), and alsoshows a state in which the laser welding apparatus 30 is in use. Asshown in this drawing, the laser welding apparatus 30 has a light source31, a convex lens 32, a planar mirror 33, and a housing 35 which housesthe components 31 through 33 and in which is formed a laser emissionport 34. The light source 31 emits a laser L. The convex lens 32converges the laser L. The planar mirror 33 reflects the laser L.

FIG. 5 is a typical view showing the shape of a pair of edge portions 21and 22 of a closed cross section rib 20. FIG. 6 is an explanatory viewshowing a process to manufacture a steel floor plate. A description willnow be given in detail of a process to manufacture a steel floor platewith reference made to these drawings.

Firstly, as is shown in FIG. 5, the pair of edge portions 21 and 22 ofthe closed cross section rib 20 are formed at an angle which enablesthem to face the surface of the deck plate 10 against which they areabutted substantially parallel therewith. The pair of edge portions 21and 22 of the closed cross section rib 20 are then abutted on thesurface of the deck plate 10 (see the arrow a in FIG. 6).

Next, the laser welding apparatus 30 irradiates a laser (see c in FIG.6) over the entire length of the closed cross section rib 20 (see thearrow b in FIG. 6) from the external side of the closed cross sectionalstructure which is formed by the deck plate 10 and the closed crosssection rib 20. As a result, the respective edge portions 21 and 22 ofthe closed cross section rib 20 are laser welded to the surface of thedeck plate. As a result of laser welding being performed on the two edgeportions 21 and 22 of the closed cross section rib 20, the closed crosssection rib 20 is joined to the deck plate 10 and a steel floor plate isthereby produced.

As shown in FIG. 4, the laser welding apparatus 30 converges the laser Lemitted by the light source 31 by means of the convex lens 32, and thenreflects the laser L which has been converged by the convex lens 32 bymeans of the planar mirror 33 so as to refract the direction of travelof the laser L. The laser welding apparatus 30 then emits the laser Lthrough the laser emission port 34. The laser L which has been emittedby the laser welding apparatus 30 is irradiated onto the deck plate 10adjacent to the edge portions 21 and 22 of the closed cross section rib20. As a result, the closed cross section rib 20 is joined to the deckplate 10. Note that, if necessary, it is possible to supply a wire W tothe vicinity of the edge portions 21 and 22 of the closed cross sectionrib 20 together with the laser L.

When laser welding is being performed by the laser welding apparatus 30,the output of the laser L, the irradiation position, the irradiationdirection, the welding speed, and the supply speed of the wire W areappropriately set. As a result, it is possible to prevent the occurrenceof melt-through, and to reduce welding residue portions to approximately0.5 mm.

Moreover, by using the laser welding apparatus 30 which is a refractivelaser welding apparatus, it is possible to freely refract the laser L toa desired angle (for example, approximately 90°), and then irradiate itacross the surface of the deck plate 10. Herewith, it is possible toirradiate easily the laser L onto the deck plate 10 adjacent to the edgeportions 21 and 22 of the closed cross section rib 20.

Furthermore, since the weld of the present embodiment is not achieved byarc welding, there are no effects from welding residual stress which iscaused by thermal contraction of the welded metal after arc welding hasended. Accordingly, there is no reduction in the assembly accuracy ofthe components, and no reduction in the tensile strength, thecompression strength, or the fatigue strength of the components due tothe effects of welding residual stress.

In addition, if, as in the present embodiment, the closed cross sectionribs 20 are welded onto the deck plate 10 without welding residualstress being generated and with only a minimal amount of welding residueportions, then stress concentration in the welding residue portions isdecreased. As a result, it is possible to reduce the extent to whichcracks are generated in an area directly above welding residue portionsin the deck plate 10. Accordingly, according to the present embodiment,it is possible to eliminate the occurrence of melt-through when weldingthe edge portions 21 and 22 of a closed cross section rib 20 onto thedeck plate 10, and to achieve an improvement in the weld quality byreducing welding residue portions to an absolute minimum. In addition tothis, it is possible to suppress the occurrence of cracks in the deckplate 10 and thereby improve the fatigue strength.

Note that in the above described embodiment the laser welding apparatus30 shown in FIG. 6 has been used, however, when implementing the presentinvention, it is also possible to use the laser welding apparatus 40(i.e., a refractive laser welding apparatus) shown in FIG. 7. The laserwelding apparatus 40 has a light source 41, a curved mirror 43 having aparaboloidal or spherical reflective surface, and a housing 45 whichhouses the portions 41 and 43 and in which a laser emission port 44 isformed.

This laser welding apparatus 40 causes a laser L which has been emittedby the light source 41 to be converged and reflected by the curvedmirror 43, and is thus able to refract the direction of travel of thelaser L. The laser L is then emitted from the laser emission port 44.

Moreover, in the above described embodiment, shaped steel having aU-shaped cross-section is used for the closed cross section ribs 20.However, when implementing the present invention, it is also possiblefor the closed cross section ribs to have a cross section of anothershape such as a V-shaped cross section, a semicircular cross section, atrapezoidal cross section, or a quadrangular cross section such as thoseshown in FIG. 8. Furthermore, in the above described embodiment, thestiffener materials were provided by the closed cross section ribs 20.However, when implementing the present invention, it is not essentialfor a closed cross-sectional structure to be formed together with thedeck plate 10, and for example, even when I-steel or the like is joinedto the deck plate 10, the present invention can be applied with superiorresults.

In addition, in the above described embodiment, a description is givenin which steel plate is used for the deck plate 10 and in which a steelfloor plate is used as an example of a stiffened plate. However, whenimplementing the present invention, the steel plate is not limited tobeing the deck plate 10 and the stiffened plate is not limited to beinga steel floor plate. If it is provided that the stiffened plate isformed by arranging stiffener materials on a steel plate, then thepresent invention can be applied with superior results.

Next, a description will be given using FIG. 9 through FIG. 11B of acase in which the welding is achieved by means of hybrid laser-arcwelding in which arc welding is performed in a process eithersimultaneously with or consecutively to the laser welding. As describedbelow, in a hybrid laser-arc welding apparatus, a laser L and an arcwelding torch M target substantially the same spot. Note that, in thedrawings, the same functional portions are given the same descriptivesymbols and any duplicated description thereof is avoided.

FIG. 9 is an explanatory view showing hybrid laser-arc welding inanother embodiment of the present invention with (a) being across-sectional view thereof and (b) being a plan view thereof. As shownin FIG. 9 (a) and FIG. 9 (b), when joining a closed cross section rib(may also be referred to below simply as a ‘rib’ or ‘stiffened plate’)20 to a deck plate (may also be referred to below simply as a ‘skinplate’ or ‘steel material’) 10 by means of hybrid laser-arc welding, thelaser L is irradiated with a distance of X from a head of the laserwelding apparatus 30 onto a weld spot. While an arc welding torch isthen applied to a position close to this irradiated portion, hybridlaser-arc welding is then performed in a welding direction Y at apredetermined welding speed. Note that it is possible for either thelaser welding or the arc welding to be given precedence.

FIG. 10 is an explanatory view showing hybrid laser-arc welding inanother embodiment of the present invention, and is an enlarged view ofa principal portion H in FIG. 9A. As shown in FIG. 10, it is preferablefor the irradiation angle θ of the laser L to be not less than 10degrees. It is also preferable for the Z target position to be setsomewhat low (for example, 5 mm or less).

When the predetermined output is less than 4 kW, it is difficult formelt-through to occur but easy for welding residue to be generated sothat the welding quality required from the present system cannot beachieved. Moreover, if the predetermined output is greater than 10 kW,since the output is large relative to the rib thickness of the steelfloor plate, there is a concern that melt-through will be generated.

On the other hand, when the predetermined welding speed is less than 50cm/min, the advantage of laser welding that is fast welding speeds islost. However, if the predetermined welding speed is greater than 200cm/min, there is a concern that welding residue will be generated.

FIG. 11A shows a state of a melt portion which corresponds to theaforementioned irradiation angle θ, and shows a melt portion Y1 when theirradiation angle θ of the laser L is not less than 10°. FIG. 11B showsa melt portion Y2 when the irradiation angle θ of the laser L is lessthan 10°. Melt-through is not generated in the melt portion Y1 in whichthe irradiation angle θ of the laser L is 10° or more, however,melt-through is generated in the melt portion Y2 in which theirradiation angle θ of the laser L is less than 10°. When theirradiation angle θ of the laser L is 10° or greater, as shown in thedrawing, since the distal end of the weld penetration is inside the deckplate 10, it is difficult for melt-through to be generated. However,when the irradiation angle θ of the laser L is less than 10°, since thedistal end of the weld penetration is positioned between the deck plate10 and the rib 20, it is easy for melt-through to be generated.

FIG. 12A and FIG. 12B show states of a melt portion corresponding to theaforementioned Z target position. FIG. 12A shows a melt portion Y3 inwhich the Z target position is 5 mm or less (i.e., low), while FIG. 12Bshows a melt portion Y4 in which the Z target position is greater than 5mm (i.e., high). In the melt portion Y3 in which the Z target positionhas been set low, melt-through is not generated, however, in the meltportion Y4 in which the Z target position has been set high,melt-through is generated. When the Z target position has been set low,as shown in the drawing, since the distal end of the weld penetration isinside the deck plate 10, it is difficult for melt-through to begenerated, however, when the Z target position has been set high,because the distal end of the weld penetration is positioned between thedeck plate 10 and the rib 20, it is easy for melt-through to begenerated.

FIG. 13 is an explanatory view showing hybrid laser-arc welding inanother embodiment of the present invention, and is an enlarged view ofa principal portion K in FIG. 9B. As shown in FIG. 11A and FIG. 11B, ina hybrid laser-arc welding apparatus, the laser L and the arc weldingtorch M as a rule aim for substantially the same spot, however,preferably, the welds of the laser L and the arc welding torch M are ata distance of 0 to 5 mm from each other. By using this hybrid laser-arcwelding, the following two advantages are obtained.

The first advantage is that hybrid laser-arc welding makes it possibleto fill the gap between the rib 20 and the deck plate 10. The gapmargins at this time are as follows.

(a) In laser welding, if there is no filler wire W, this gap margin islimited to approximately 0.5 mm.(b) In laser welding, if there is filler wire W, this gap margin islimited to approximately 1 mm.(c) In hybrid laser-arc welding, the gap may be as much as 3 mm.

The second advantage is that, compared with when only laser welding isused, because there is also heat input from the arc welding, rapidheating and rapid cooling can be avoided. Because of this, the hardnessof the welding metal is less than when laser welding (i.e., using fillerwire W) is used.

Moreover, in electronic beam welding and laser welding, fundamentally, ajoin is achieved by only welding the base materials without adding anywelding material, however, in cases such as those described below, it ispreferable to add welding material (referred to below as ‘filler wire’or ‘wire’) W.

(a) When there is a gap in the weld portion in a piece of work and it isnecessary to provide a supplementary material in order to achieveconsistent bead formation.(b) When it is necessary to improve the cleanliness and mechanicalproperties of a weld metal.(c) When weld reinforcement is being formed in a material whose natureis such that surface beads are easily hollowed out therein (such asaluminum alloy and the like) when performing complete-melt welding.

Moreover, in laser welding, the greatest advantage in using the fillerwire W is that it is possible to fill the gap between the stiffenedplate (i.e., rib) 20 and the steel plate (i.e., skin plate or deckplate) 10. Note that the present invention can also be applied to a deckplate 10 having a curved surface structure (not shown). It is alsopossible for the present invention to be applied to a bevel shape inwhich there is no gap with the deck plate 10 (see FIG. 5 and FIG. 7).

In addition, the method which is most commonly used to add a weldingmaterial in laser welding is to gradually feed out the filler wire W, aspartially shown in FIG. 7. Because both an electron beam and a laserbeam which are converged on a weld portion have extremely smalldiameters, it is common that a wire having a narrow diameter of 1 mm orless is used for the wire W. This filler wire W is fed precisely via awire guide (not shown) to the beam irradiation point where it is thenmelted.

Note that the present invention can be applied to a wide range ofcross-sectional shapes for the ribs 20 such as U-shapes and circularshapes and the like.

1. A stiffened plate which is formed by welding a plurality of stiffenermaterials which, together with a steel plate, form closedcross-sectional structures onto a surface of the steel plate, whereinthe stiffener materials are joined to the steel plate as a result ofedge portions of the stiffener materials which are in contact with thesteel plate by being laser welded at a predetermined welding speed as aresult of a laser having a predetermined output being irradiated from apredetermined direction thereon from the external side of the closedcross-sectional structures.
 2. The stiffened plate according to claim 1,wherein the edge portions of the stiffener materials are formed so as toface the surface of the steel plate substantially parallel therewith,and are laser welded to the steel plate.
 3. The stiffened plateaccording to claim 1, wherein the stiffener materials are closed crosssection ribs which have a substantially U-shaped cross sectionorthogonally to the longitudinal direction.
 4. The stiffened plateaccording to claim 1, wherein, instead of laser welding, the stiffenermaterials are welded to the steel plate by means of hybrid laser-arcwelding in which laser welding and arc welding are performed incombination.
 5. A method of manufacturing a stiffened plate which isformed by welding a plurality of stiffener materials which, togetherwith a steel plate, form closed cross-sectional structures onto asurface of the steel plate, wherein the steel plate and the stiffenermaterials are joined together as a result of edge portions of thestiffener materials which are in contact with the steel plate by beinglaser welded at a predetermined welding speed as a result of a laserhaving a predetermined output being irradiated from a predetermineddirection thereon from the external side of the closed cross-sectionalstructures.
 6. The method of manufacturing a steel plate according toclaim 5, wherein, prior to the laser welding, the edge portions of thestiffener materials are formed so as to face the surface of the steelplate substantially parallel therewith.
 7. The method of manufacturing asteel plate according to claim 5, wherein, in the laser welding, arefractive laser welding apparatus which refracts laser light which hasbeen converged by a convex lens onto a planar mirror is used toirradiate laser light over the surface of the steel plate so as to jointhe stiffener materials to the steel plate.
 8. The method ofmanufacturing a steel plate according to claim 5, wherein, in the laserwelding, a refractive laser welding apparatus which converges andrefracts laser light by using a parabolic mirror or spherical mirror isused to irradiate laser light over the surface of the steel plate so asto join the stiffener materials to the steel plate.
 9. The method ofmanufacturing a steel plate according to claim 5, wherein the welding isachieved by hybrid laser-arc welding in which arc welding is performedin a process either simultaneously with or consecutively to the laserwelding.
 10. The method of manufacturing a steel plate according toclaim 5, wherein, in the laser welding or in the hybrid laser-arcwelding, filler wire is used.
 11. The method of manufacturing a steelplate according to claim 5, wherein, in the laser welding or in thehybrid laser-arc welding, a bevel shape in which there is no gap withthe skin plate is used.
 12. The method of manufacturing a steel plateaccording to claim 5, wherein, in the laser welding or in the hybridlaser-arc welding, the skin plate is used in a structure having a curvedsurface.